UNIVERSITY    OF    CALIFORNIA     AGRICULTURAL    EXPERIMENT   STATION 

BENJ.    IDE    WHEELER,    PRESIDENT 

COLLEGE   OF  AGRICULTURE 

Vr     «va",v""'1-'  v»nc  THOMAS    FORSYTH    HUNT,    Dean  »nd  Director 

BERKELEY  H.    E.    VAN    NORMAN,    Vice-director    and    Dean 

University  Farm  School 


CIRCULAR  No.  161 
March,  1917 

POTATOES  IN  CALIFORNIA 

By  J.  W.  GILMOEE 


The  principal  regions  of  California  in  which  potatoes  are  grown 
at  the  present  time,  are  the  Delta  region,  comprising  parts  of  San 
Joaquin,  Sacramento,  and  Contra  Costa  counties;  the  Salinas  Valley 
in  Monterey  County;  the  Napa  and  Sonoma  Valleys  in  their  respec- 
tive counties,  and  the  regions  comprising  parts  of  Santa  Barbara, 
Los  Angeles,  and  Riverside  counties.  The  average  yield  per  acre  in 
these  several  localities  differs  widely,  ranging  from  4500  to  12,000 
pounds.  In  general  where  soil  conditions  are  optimum  the  yields  vary 
according  to  the  mean  temperature  of  the  growing  season;  the  best 
yields  being  obtained  in  localities  with  relatively  cool  summer  tem- 
peratures. Hence,  the  interior  valleys  are  not  so  well  adapted  for 
potatoes  as  the  cooler  coast  sections.  It  should  be  noted,  however, 
that  potatoes  are  grown  in  all  counties  of  the  state,  and  by  improved 
culture  methods  the  potato  acreage  can  be  extended. 

Soil  Considerations. — In  order  to  meet  the  requirements  for  profit- 
able potato  yields  a  soil  must  be  superior  in  three  respects — drainage, 
organic  matter  content,  and  preparation.  Of  these  three,  granted 
that  there  is  reasonable  fertility  present,  drainage  is  most  important. 
The  good  potato  soils  in  all  parts  of  the  country  are  those  that  are 
either  naturally  or  artificially  well  drained.  On  the  other  hand, 
there  are  good  soils  in  potato-producing  regions  which  are  not  fit 
for  potatoes  because  they  are  not  well  drained.  Even  though  a  soil 
be  heavy  in  texture,  it  will  become  friable  and  productive  if  it  is 
well  drained  and  contains  a  fair  amount  of  organic  matter. 

The  problems  of  good  drainage  and  organic  matter  content  are 
inter-related,  because  they  both  have  to  do  with  affording  the  growing 
crop  with  a  sufficient  and  constant  supply  of  moisture.  This  is  a 
very  important  consideration,  for  tubers  do  not  develop  well  under 


conditions  of  a  fluctuating  moisture  supply.  But  in  addition  to  aid- 
ing in  the  maintenance  of  a  sufficient  and  constant  supply  of  moisture, 
organic  matter  ameliorates  temperature  conditions  to  the  end  of 
improving  both  yield  and  quality  of  the  crop.  A  soil  that  is  deficient 
in  organic  matter  will  be  of  poor  physical  condition  for  potatoes,  will 
vary  in  moisture  content  during  the  growing  season,  and  will  become 
warm  and  cool  as  the  temperature  of  the  air  changes. 

Preparation. — The  importance  of  early  and  thorough  preparation 
cannot  be  overemphasized.  It  is  estimated  that  this  procedure  gives 
half  the  assurance  of  the  crop.  The  land  should  be  plowed  as  deeply 
as  possible  in  the  fall  and  allowed  to  remain  rough  during  the  winter, 
in  order  that  it  may  be  benefitted  by  the  winter  rains  and  weather. 
As  early  in  the  spring  as  possible  it  should  be  worked  with  disc  and 
harrow  as  often  as  may  be,  in  order  to  keep  the  surface  in  good  con- 
dition and  to  destroy  early  germinating  weeds.  It  is  desirable  that 
the  interval  between  opening  the  furrows  before  planting  and  closing 
them  afterward  shall  be  as  short  as  possible.  In  case  the  land  cannot 
be  plowed  until  spring  it  should  be  plowed  deeply  and  worked  fre- 
quently, in  order  to  give  it  sufficient  compactness  to  facilitate  the 
distribution  and  retention  of  moisture. 

The  Seed  Set. — The  yield  of  potatoes  on  reasonably  good  soil  will 
increase  with  the  size  of  the  set.  But  there  is  a  limit  in  size  above 
which  the  cost  of  the  seed  will  be  greater  than  the  value  of  the  extra 
yield  and  below  which  the  yield  of  marketable  tubers  will  not  pay 
for  the  growing  of  the  crop.  The  results  of  experiments,  extended  in 
time  and  place,  indicate  that  the  seed  set  producing  the  greatest  re- 
turns over  the  cost  of  seed  and  of  growing  the  crop  should  be  between 
two  and  three  ounces. 

Whole  or  Cut  Sets. — The  best  results  have  followed  the  use  of  cut 
sets.  For  optimum  yields  the  size  need  not  be  so  large  for  whole 
tubers  as  for  those  cut.  It  has  been  found  that  whole  tubers,  when 
size  is  equal  to  cut  sets,  often  give  somewhat  better  results  than  cut 
sets.  This  is  because  the  cut  sets  lose  something  by  evaporation  and 
contact  of  the  fresh  surface  with  the  soil,  are  more  subject  to  attack 
of  diseases  in  the  soil,  and  also  because  there  are  more  eyes  on  the 
whole  tuber.  Hence,  the  whole  tuber  has  a  better  chance  of  setting 
forth  its  strongest  eyes  as  sprouts. 

On  the  other  hand,  the  planting  of  whole  tubers  is  more  often 
questionable  because  the  small  whole  tuber  is  likely  to  represent  the 
progeny  of  a  weak  producing  hill.  For  example,  in  a  crop  of  potatoes 
where  18  per  cent  of  the  total  crop  were  unmarketable  or  culls,  it  was 
found  that  76  per  cent  of  these  culls  were  from  weak  or  unproductive 


hills.  These  culls  were  planted  in  comparison  with  cut  sets  of  the 
same  size  from  large  tubers.  The  yield  of  the  culls  was  only  56  per 
cent  of  the  yield  from  the  cut  sets.  It  follows,  therefore,  that  the 
safe  practice  is  to  plant  cut  sets  from  large  tubers  rather  than 
small  whole  tubers,  unless  they  represent  productive  hills.  If,  how- 
ever, the  whole  tubers,  though  small,  are  from  productive  hills,  these 
may  be  better  for  an  additional  reason :  that  small  tubers  are  likely 
to  be  less  mature  than  large  tubers ;  and  after  the  tubers  have  passed 
through  the  required  rest  period,  stronger  sprouts  emanate  from 
immature  than  from  mature  tubers.  This  is  the  reason  why  stronger 
sprouts  come  from  the  bud  end  than  from  the  stem  end.  This  also 
explains  why  second  crop  potatoes  are  often  better  for  seed  than  those 
of  the  main  crop. 

Cutting. — Since  the  strongest  buds  are  near  the  bud  end  of  the 
tuber  the  seed  set  should  be  cut  in  such  a  manner  as  to  include  one 
or  two  of  these  strong  buds  on  each  piece.  With  this  in  mind  the 
tubers  should  be  cut  so  as  to  expose  as  little  cut  surface  as  possible. 
Usually  a  diagonal  cut  rather  than  a  straight  lengthwise  or  crosswise 
cut  gives  best  results.  Skill  in  this  work  is  gained  only  by  experience 
and  when  one  is  expert  a  decision  as  to  how  to  cut  can  be  made  while 
the  tuber  is  brought  by  the  one  hand  to  the  knife  in  the  other. 

There  are  mechanical  cutters  on  the  market,  but  these  do  not 
perform  good  service,  for  there  is  much  waste  from  small  pieces  which 
often  possess  strong  eyes. 

Seed  potatoes  are  often  infected  with  diseases.  The  presence  of 
these  can  generally  be  detected  by  cutting  off  a  small  portion  of  the 
stem  end.  If  there  appears  either  in  the  center  or  around  the  edge  a 
spot  or  ring  of  brown,  the  tuber  should  be  cast  out,  for  this  is  likely 
to  be  the  fusarium  wilt  and  will  be  transmitted  to  the  growing  crop. 

It  is  a  widespread  practice  to  sprinkle  some  dry,  dusty  material 
over  the  tubers  when  cut.  The  substances  usually  used  for  this  pur- 
pose are  gypsum,  lime,  ashes,  and  road  dust.  Neither  experience  nor 
experiment  has  shown  any  decided  advantage  in  this  practice  except 
under  those  conditions  when  the  addition  of  one  of  these  substances 
materially  benefits  the  soil ;  or  when  the  cut  tubers  may  not  be  planted 
for  several  days  after  they  are  cut.  Gypsum  shows  a  benefit  more 
often  than  the  other  substances,  but  this  is  because  nearly  all  soils  are 
benefitted  by  gypsum.  Likewise  ashes  give  good  results  when  the 
soil  requires  potash.  One  must  use  lime  with  care,  however,  for  lime 
promotes  conditions  in  the  soil  favorable  to  the  growth  of  disease 
fungi,  hence  the  crop  may  be  injured  more  by  disease  than  benefitted 
by  the  lime.  Seed  sets  should  be  planted  as  soon  after  cutting  as 
possible,  within  twenty-four  hours  at  most. 


Sprouting. — It  is  a  good  practice  when  possible  to  sprout  seed 
tubers  before  cutting  and  planting.  This  is  usually  done  by  placing 
the  dormant  tubers  in  diffused  or  subdued  light  for  about  three  weeks 
before  planting.  Sometimes  it  is  well  to  cover  the  tubers  with  a  light 
layer  of  moist  straw,  especially  if  they  are  placed  out  of  doors.  By 
this  process  short,  stubby  and  somewhat  green  sprouts  about  one- 
fourth  to  three-eighths  inch  long  grow  out  from  several  eyes.  They 
are  not  so  easily  broken  off  as  if  sprouted  in  the  dark.  There  are 
several  advantages  in  sprouting  as  suggested.  In  the  first  place  only 
those  tubers  showing  vigorous  sprouts  need  be  selected  for  planting, 
thus  a  more  perfect  stand  is  assured,  and  a  more  uniform  growth  is 
also  secured.  In  the  second  place,  recognizing  that  rhizomes  come 
out  only  at  the  nodes  or  joints  of  the  sprout  underground,  by  sprout- 
ing, more  nodes  will  form  beneath  the  surface  than  if  the  tubers 
begin  their  sprouting  in  the  ground.  Thus,  the  yield  of  tubers  is 
likely  to  be  increased.  Another  advantage  is  that  there  may  be 
additional  time  to  finish  preparing  the  land  if  that  has  not  been 
completed  for  an  early  planting. 

Planting. — If  the  soil  is  rich,  deep,  well  drained  and  well  pre- 
pared, potatoes  may  be  planted  as  close  as  12  inches  in  the  row, 
with  rows  20  inches  apart.  But  under  usual  soil  conditions  in  Cali- 
fornia it  seems  best  to  plant  from  12  to  14  inches  in  the  row  with 
rows  36  to  42  inches  apart.  In  this  manner  the  crop  may  be  cultivated 
by  horse  labor  without  seriously  disturbing  the  vines  in  their  position 
and  growth.  The  wider  distance  is  essential  under  conditions  where 
soil  is  poor  and  the  moisture  supply  inadequate. 

The  best  depth  for  potatoes  under  normal  conditions  is  four  to 
five  inches.  If  planted  too  deep  the  tubers  will  be  small  and  of 
poor  quality.  If  planted  too  shallow  they  will  be  of  variable  size, 
unshapely  and  often  greened  by  exposure  to  the  light.  In  heavier 
soils  the  shallower  depth  may  not  be  exceeded,  but  in  lighter  soils  the 
greater  depth  may  be  followed.  There  are  several  makes  of  machines 
on  the  market  for  planting.  The  main  advantage  in  their  use  is  in 
their  enabling  one  to  plant  the  largest  possible  area  in  good  season. 
Their  main  defect  is  that  they  do  not  work  the  soil  thoroughly  and 
deeply,  and  in  poorly  prepared  soils  the  tubers  are  likely  to  be  planted 
too  shallow. 

Time  of  Planting. — The  optimum  time  of  planting  potatoes  in 
California  extends  over  a  considerable  period.  In  the  coast  regions 
of  southern  California  the  planting  may  begin  in  the  early  winter 
months,  especially  for  crops  that  are  designed  for  the  early  market. 
In  most  regions,  however,  the  planting  may  begin  in  early  March  and 


continue  until  June,  the  prospects  for  largest  yields  diminishing  as 
the  season  advances.  For  the  second  crop,  the  planting  is  usually 
begun  in  late  July  or  early  August. 

There  is  a  general  correlation  between  the  yield  and  the  period 
of  maturation.  The  longer  the  growing  season  the  greater  the  yield. 
Early  potatoes  require  from  70  to  90  days  to  bear  a  crop,  while  the 
late  varieties  require  from  130  to  150  days.  It  is  recognized  that 
potatoes  are  good  food  at  any  stage  of  growth,  but  immature  potatoes 
do  not  possess  good  keeping  or  shipping  qualities. 

Culture. — If  the  land  is  deep,  rich,  and  well  prepared,  and  if 
the  seed  sets  have  been  well  chosen  and  prepared  and  seasonably 
planted,  little  more  can  be  done  to  insure  a  crop.  Cultivation  may 
be  given  as  the  conservation  of  moisture  or  the  destruction  of  weeds 
may  require.  Usually  three  or  four  cultivations  are  as  many  as  the 
crop  will  pay  for.  The  first  cultivations  may  be  deep,  but  the  later 
treatments  should  be  shallow,  because  of  the  danger  of  disrupting 
the  shallow  feeding  rootlets.  It  is  best  to  cultivate  potatoes  level, 
but  under  conditions  where  water  is  excessive  they  may  be  ridged 
in  order  to  secure  drainage.  Under  conditions  where  the  crop  has 
to  be  grown  entirely  by  irrigation  they  may  also  be  ridged  and 
especially  if  the  rows  are  close  together,  for  it  is  inimical  for  free 
water  to  come  in  contact  with  the  growing  tuber  for  any  length  of 
time. 

Irrigation. — Under  most  conditions  where  irrigation  is  necessary 
it  has  been  found  that  about  1%  acre  feet  of  water  are  necessary  to 
produce  a  good  yield.  This  amount  should  be  put  on  in  three  or 
four  applications,  beginning  when  the  plants  are  approaching  full 
growth  and  repeated  at  intervals  of  fourteen  to  eighteen  days  until 
the  vines  show  signs  of  maturity.  A  light  cultivation  should  be  given 
after  each  irrigation  to  break  the  crust  in  the  furrow  and  conserve  the 
water  applied.  The  water  should  be  applied  by  furrows  rather  than 
by  flooding.  Sometimes  in  light  soils  a  furrow  between  every  alter- 
nate pair  of  rows  is  sufficient,  but  a  furrow  between  every  two  rows 
is  best.  The  potato  does  not  thrive  under  a  fluctuating  moisture 
supply,  hence,  every  attempt  should  be  made  by  irrigation  and  culti- 
vation to  keep  the  moisture  supply  sufficient  and  constant. 

Diseases. — There  are  many  diseases  attacking  the  potato  in  Cali- 
fornia, and  because  of  our  mild  climate  and  other  conditions  favorable 
to  development  they  are  often  difficult  to  control. 

Perhaps  the  most  destructive  disease  is  the  Fusarium  wilt.  This 
pest  may  be  detected  by  a  brown  spot  or  ring  appearing  on  the  fresh 
surface  of  the  stem  and  of  the  tuber  when  cut.    Such  tubers  for  seed 


should  be  cast  out.  If,  however,  they  are  planted,  or  if  the  disease 
is  already  in  the  soil  it  manifests  itself  by  a  sudden  wilting  of  the 
plant  as  it  comes  into  vigorous  growth.  The  plant  soon  turns  brown 
and  dies.  When  this  disease  becomes  established  in  the  soil  six 
or  seven  years  are  required  to  eradicate  it,  during  which  time  no 
host  plants  should  be  grown  upon  the  land.  The  remedial  measures 
against  this  disease  are,  therefore,  rotation  of  crops  and  the  choice 
of  disease-free  seed. 

The  disease  second  in  importance  is  Rhizoctonia.  The  presence  of 
this  disease  may  be  detected  by  small  wart-like  and  dirty  specks,  or 
excrescences,  on  the  surface  of  the  tuber.  It  manifests  itself  by  a 
yellowing  and  wilting  of  the  growing  plants  as  they  approach  full 
size.  The  ravages  of  this  disease  may  be  lessened  to  a  considerable 
extent  by  treating  the  seed  tubers  in  a  solution  of  formalin,  one  pound 
to  thirty  gallons  of  water  for  one  and  one-half  to  two  hours  according 
to  the  prevalence  or  intensity  of  the  disease.  If  the  tubers  have 
sprouted  before  they  can  be  treated  it  would  be  well  to  treat  them 
in  a  solution  of  corrosive  sublimate  (mercuric  chlorid),  four  ounces 
to  thirty  gallons  of  water,  for  one  and  one-half  hours.  This  treatment 
does  not  injure  the  sprouts  as  formalin  does.  It  must  be  remembered, 
however,  that  corrosive  sublimate  is  an  active  poison  and  every  pre- 
caution should  be  taken  in  the  interests  of  plants  and  animals. 

Scab. — Oospora:  This  disease  manifests  itself  by  large  dried  or 
scabby  spots  occurring  on  the  tuber,  both  before  and  after  they  are 
dug.  The  appearance  on  the  plant  is  not  so  marked  as  with  the 
diseases  previously  mentioned;  but  the  yield  from  infested  fields  is 
materially  reduced.  The  treatment  for  seed  tubers  supposed  to  be 
infested  is  the  same  as  for  Rhizoctonia  mentioned  above.  But  it  does 
not  seem  feasible  by  the  evidence  of  experiments  to  treat  tubers  for 
planting  in  soil  that  is  already  infested  with  either  disease.  One 
pound  of  formalin  will  treat  from  three  to  five  sacks  of  potatoes, 
according  to  the  intensity  of  the  diseases,  or  the  prevalence  of  dirt 
on  the  tubers. 

Another  disease  that  is  very  prevalent  and  destructive  to  potatoes 
in  the  Delta  region  is  that  commonly  called  Leaks  (Rhizopus  nigri- 
cans). This  disease  manifests  itself  after  the  tubers  are  dug  and 
usually  occurs  while  the  tubers  are  piled  in  sacks  on  the  levees  await- 
ing shipment.  The  effect  is  to  cause  the  potatoes  to  collapse,  the  water 
running  away  from  a  large  pile  in  a  stream.  Sometimes  the  disease 
does  not  manifest  itself  until  the  tubers  are  in  transit  or  in  storage. 

The  disease  attacks  only  those  tubers  that  have  been  broken, 
chipped,  or  abraded  in  digging  and  handling.    The  trouble  cannot  be 


absolutely  prevented  or  cured,  once  it  has  made  its  attack,  but 
precaution  and  care  can  be  exercised  in  the  digging  and  handling  to 
avoid  bruising  or  breaking  the  skin. 

Of  the  several  insects  that  attack  potatoes  the  most  noteworthy  are 
the  flee  beetle,  which  punctures  the  leaves,  thus  inviting  and  favoring 
the  attack  of  fungi ;  the  eel  worm,  which  occurs  in  rich  soils  and  finds 
in  the  growing  potato  a  rich  supply  of  food ;  and  the  tuber  moth,  the 
larva  of  which  eats  holes  and  cavities  in  the  tuber. 

The  flee  beetle  cannot  be  entirely  prevented  from  attacking  the 
potato,  but  it  can  be  discouraged  by  sprays  of  bordeaux  mixture.  To 
combat  the  eel  worm  it  is  necessary  to  plant  clean  seed  sets,  or  if  the 
soil  is  already  infested  to  plant  other  plants  non-attractive  to  the  pest 
until  the  soil  is  rid  of  it. 

Mulching. — In  some  parts  of  the  state,  especially  in  the  interior 
valleys,  where  the  summers  are  warm,  potatoes  do  not  grow  to 
desirable  size.  Encouragement  and  advantage  may  be  offered  by 
mulching  the  ground  with  broken  or  partly  decayed  straw,  or  other 
light  waste  material.  This  mulch  should  be  applied  after  the  vines 
are  well  along  in  their  growth.  If  the  ground  has  been  well  prepared 
and  early  cultivations  are  given  this  mulch  may  serve  in  lieu  of  late 
cultivations.  The  effect  of  the  mulch  is  to  keep  the  ground  cooler 
and  more  constantly  moist  than  it  would  otherwise  be,  thus  simu- 
lating soil  conditions  in  more  northern  latitudes. 

Fertilizing  Potatoes. — There  are  few  experimental  data  for  Cali- 
fornia bearing  on  the  question  of  the  use  of  fertilizers.  It  should 
be  noted,  however,  that  the  benefits  from  fertilizers  depend  upon 
several  factors,  prominent  among  which  may  be  mentioned  the  type 
and  character  of  the  soil  and  the  amount  of  moisture  available.  In 
general  where  fertilizers  are  desirable  a  mixture  containing  2  to  3 
per  cent  of  nitrogen,  6  to  8  per  cent  of  phosphoric  acid,  and  8  to  10 
per  cent  of  potash,  will  probably  give  good  results.  A  mixture  of 
forty  pounds  of  sulfate  of  ammonia,  120  pounds  of  acid  phosphate 
and  eighty  pounds  of  sulfate  of  potash  per  acre  will  be  sufficient  for 
all  but  very  light  soils.  The  mixture  should  be  applied  in  the  furrow 
and  stirred  in  the  soil  before  planting. 

Varieties. — While  many  varieties  of  potatoes  have  been  tried  in 
California,  but  few  have  been  found  well  adapted.  The  following 
have  been  found  of  some  merit  in  various  parts  of  the  state : 

Early. — Chili :  A  pink,  smooth-skin  potato,  adapted  to  the  north- 
ern Sacramento  Valley  and  coast  sections. 

Early  Ohio:  A  pink-skin  tuber,  grown  largely  in  the  southern 
part  of  the  state. 


8 

Early  Rose :  White  and  red.  Adapted  to  a  wide  range  of  con- 
ditions and  used  both  for  a  first  and  second  crop. 

Medium. — American  Wonder:  A  white,  shapely  potato  of  good 
yielding  and  cooking  quality. 

British  Queen :    A  white  potato  which  may  be  used  also  for  late. 

Late. — Burbank:  The  most  widely  grown  potato  in  the  state.  It 
has  a  tendency  to  grow  too  large  and  long,  and  if  not  carefully  grown 
its  eyes  will  become  rather  prominent  and  deep. 

There  are  a  number  of  other  varieties  which  may  be  good  for 
certain  localities  and  conditions,  such  as  Triumph,  Pearl,  and  Uncle 
Sam,  but  those  mentioned  have  been  tried  extensively  and  perhaps 
are  more  certain  one  year  with  another. 

Cost  of  Production. — The  following  figures  represent  as  nearly  as 
can  be  presented,  average  costs  of  the  various  stages  in  potato  pro- 
duction. In  some  instances  the  range  is  wide.  The  figures  under 
market  value  represent  average  for  several  years  past  and  do  not  refer 
to  prices  at  present  prevailing. 

Preparing  land  and  planting — 

Plowing  and  preparing  land  ($2.50-$8.00)    (Coast  sections)   $  5.00  per  acre 

Plowing  for  planting   ($0.00-$3.00)    (Delta  sections)  1.50 

Seed   ($2.00  per  sack)  12.00 

Preparing    seed — cutting    1.00 

Preparing    see — disinfecting    .35 

Planting  (six  acres  per  day)  ... 90 

Growing  tire  crop — 

Cultivating    ($0.00-$2.00)    1.00 

Hoeing    ($0.00-$2.00)    1.00 

Irrigation    ($0.00-$10.00)    6.00 

Harvest — 

Digging    (machine)    ($1.00-$200)  ! 1.50 

Picking  up  and  sacking    (after  machine)  06  per  cwt. 

Digging,  picking  up  and  sacking  (by  hand)    (10c-20c) 15  per  sack 

Sacks     10 

Hauling  off    (five  miles)  1.00  per  ton 

Covering  sacks  (on  levee  or  in  field)  10 

Hauling  to  levee    (Delta) 50 

Market  Value  of  Yield — 

High    $2.50  (lsts)  per  cwt. 

1.75  (2nds) 

Low  .65  (lsts) 

.50  (2nds) 

Average    1.00  (lsts) 

.65  (2nds) 
Culls    15 


